The various 52mm drop-ins DX (& others) sell will fit a Maglite with some slight modding (grinding lip of Maglite head & lip of drop-in module). You can choose to run 18650's or 26650's. Use 3/4" & 1" PVC water pipe or electric conduit for battery spacers (3/4" inside the 1" for 18650 use).
I built a 2D Mag build with the single XM-L DX drop-in running on (2) 18650's. Beamshots here. Not a difficult build if you have a grinder/dremel/files. I then wired direct off the driver to the ground and switch contacts (i.e. I didn't use the switch tower like some do).
Be careful getting the Mag switch out without damaging it (I destroyed mine). The newer Mags use a T8 torx bit, but the older ones used a 5/64" allen wrench.
Oh, if you want to go NiMh, you can use 3AA or 4AA carriers (perhaps modded to copper contacts), but with most batteries you have to bore the light for them to fit. Oh, you can also squeeze 3 C NiMh into a 2D Mag using a PVC spacer and shortening the tailcap spring (drop the spring down into the bottom of the tailcap).
And do NOT expect to get ground from the body to drop-in contact - won't work. Best to solder a wire from the driver ground to the ground screw contact on the switch. I've also soldered to the c-ring (retaining ring which keeps the switch from moving forward) after grinding the body & c-ring for good contact.
That link for the 5 XR-E’s is a good drop in. Best over-all output light I own. I used 2 32650’s with 5500mAh capacity for super long runtime. Slightly broadened the lip in the tail for the battery to fit into, seated the spring to the bottom after cleaning the anodizing out so it would indeed make ground. Cut the tower, wired the module directly to the switch.
I used a dremel to grind the lip of the module and get the bezel to screw down further so the o-ring would seal better. No fancy tools, not a machinist. Great mod!